When you import your photos into Lightroom, they are not actually converted from Gamma 2.2 to the Gamma 1.80 of Lightrooms ProPhoto RGB space. Lightroom 4 does not always use ProPhoto RGB as it’s internal color space as is widely believed. In the Library module for example, it uses Adobe RGB color space to preview everything. The Develop module also displays in the Adobe RGB color space. The only way to escape working in Adobe RGB is to actually select a Soft Proof profile. So Lightroom is using a gamma of 2.2 to display all your images to you, unless you select a Soft Proof profile that differs from Gamma 2.2 or from Adobe RGB.